The "Unable to write entry" error message appear when you have a problem of permission to create or update files and folder on your system as explained in FAQ trouble shooting
/faq#sec-14
Sometime, you can solve this kind of problem when using the FTP Layer present in Joomla / Global configuratoin.
With the FTP Layer, this may increase the permission when the user and password you use for the FTP connection has the permission on the files.
See also my other reply to this question in the form
/forum?func=view&id=67&catid=7#67